This application claims the benefit of Korean Application No. 99-29278, filed Jul. 20, 1999, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a phase change optical disc, the phase control layer of which has an optical characteristic varying with laser beam irradiation, and more particularly, to a phase change optical disc, the resolving power of which is improved to allow high density optical recording, even if the size of a recording mark is reduced as the density of the optical disc increases.
2. Description of the Related Art
Phase change optical discs are optical information recording media on which information can be written, and from which information can be reproduced and erased by irradiation with laser beams. For phase change optical discs, an optical head can easily be constructed, and simultaneous recording and removal (i.e., overwriting) can be easily performed. For this reason, there has been research and investigation into increasing the density of a phase change optical disc.
FIG. 1 is a vertical cross sectional view of a typical phase change optical disc. Referring to FIG. 1, a conventional phase change optical disc 10 has a multilayer structure in which a first dielectric layer 12, a phase change recording layer 13, which is formed of a phase change material whose optical characteristic varies with irradiation of a recording beam, a second dielectric layer 14 and reflective layer 15, are sequentially laminated on a transparent substrate 11, which is formed of, for example, acrylic acid resin or polycarbonate (PC).
In this multilayer structure, the first and second dielectric layers 12 and 14 are typically formed of ZnSxe2x80x94SiO2. The phase change recording layer 13 is formed of Ge2Sb2Te5. The reflective layer 15 is formed of aluminum (Al) or an aluminum alloy.
Information is recorded on or reproduced from this conventional phase change optical disc 10 having a multilayer structure. This is based on its characteristic that a part of the phase change recording layer 13, on which a laser beam is applied, becomes a crystalline or amorphous state depending on the power of the incident laser beam and the cooling speed, and thus the optical characteristic of the phase change recording layer 13 is changed. Information is recorded by generating a recording mark by melting the phase change recording layer 13, which is initialized to a crystal phase by laser beam irradiation, using a recording pulse of high power and then rapidly cooling it into an amorphous state. Since the reflectance at the crystal phase change recording layer 13 when a laser beam is incident on the optical disc is different from that at the amorphous phase change recording layer 13 when a laser beam is incident on the optical disc, the information of a recording mark is reproduced as an electrical signal by a photodetector, which detects the difference between the reflectances. On the other hand, information is erased by removing a recording mark by crystalizing the amorphous recording mark using an erasing pulse of low power.
In such a phase change optical disc, the size of a recording mark becomes smaller as the recording density increases. Where the size of a recording mark becomes smaller, crosstalk occurs between adjacent marks during reproduction of a signal, thus deteriorating the characteristics of the reproduced signal. Moreover, the resolving power for reproducing the signal becomes poor.
The resolving power for reproducing the signal of a phase change optical disc depends on the wavelength (xcex) of a laser beam and numerical aperture (N.A.) of an object lens in an optical system. The characteristic of the reproduced signal is bad when the length of a recording mark (or a pit) formed on an optical disc is smaller than the value of xcex/(2 N.A.), indicating a diffraction limit. The characteristic of the reproduced signal is good when the length of a recording mark (or a pit) formed on an optical disc is greater than the value of xcex/(2 N.A.), indicating a diffraction limit. Accordingly, it is required to decrease the wavelength of a laser beam and to increase the numerical aperture of an objective lens for the purpose of recording information on a phase change optical disc at a high density.
However, there is a limit in decreasing the wavelength of a laser beam and in increasing the numerical aperture of an object lens. Particularly, a lens having little optical aberration is required to increase the numerical aperture of an object lens, but it is difficult to manufacture such a lens practically. Moreover, the lens having little optical aberration may cause skew in a disc and vibration, spoiling the stability of focus.
Accordingly, what is desired is a high resolution reproduction method for reproducing a signal of good quality by eliminating the signal of an adjacent mark, which is mixed into the signal of a pertinent mark, thereby realizing high density in a phase change optical disc.
To solve the above problem, an object of the present invention is to provide a high density phase change optical disc for obtaining a playback signal of good quality by eliminating the signal of an adjacent mark, which is mixed into the signal of a pertinent mark when optical information is reproduced from the optical recording medium, even if the size of a recording mark is reduced for realizing a high density optical medium.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above object of the invention, there is provided a phase change optical disc including at least one first dielectric layer thinly formed on a transparent substrate; a phase change recording layer which converts between a crystal phase and an amorphous phase by irradiation with a recording beam; a reflective layer; and a phase control layer disposed between the substrate and the phase change recording layer, the phase control layer having two areas in which the irradiation with a reproduction beam causes a phase difference that alters the optical path of the reflected reproduction beam passing through the areas, the two areas being defined in a laser spot.
In a preferred embodiment of a phase change optical disc according to the present invention, on the transparent substrate are sequentially laminated the first dielectric layer, the phase control layer having two areas in which the irradiation with a reproduction beam causes a phase difference that alters the optical path of the reflected reproduction beam passing through the areas, the two areas being defined in a laser spot, a second dielectric layer, the phase change recording layer, which converts between a crystal phase and an amorphous phase by irradiation with a recording beam, a third dielectric layer, the reflective layer, and a protective layer.
It is preferable that the phase difference of the reflected reproducing beam caused by irradiation of the phase control layer substantially has a minimum value of 0 degrees in one of the two areas defined on the phase control layer, and a maximum value of 180 degrees in the other area. The phase control layer is formed of a phase change material which converts between a crystal phase and an amorphous phase or converts from a crystal phase of one structure to a crystal phase of another structure. Preferably, the phase control layer is formed of a material selected among the GeSbTe family, InSbTe family, AgInSb family, Au, and Ni.